She brought it in a sandwich bag. That is how I usually find out — not a question in the history, but a resealable bag of hair produced somewhere around minute eleven of the appointment, with an expression I have come to recognise.
It is one of the few side effects people are genuinely frightened of, and one of the few where the honest answer is more reassuring than the internet’s.
What telogen effluvium actually is
Hair follicles cycle. Most are in an active growth phase at any moment; a minority are in a resting phase that ends with the hair being shed. Normally this is staggered, and you lose fifty to a hundred hairs a day without noticing.
A significant physiological stressor pushes an abnormally large cohort of follicles into the resting phase simultaneously. Two to four months later they all shed at once. It is diffuse — thinning across the whole scalp, more hair in the shower drain and the brush — rather than patchy or receding.
That delay is the reason people misattribute it. The trigger was month one. The shedding is month four. By then you have been on the drug long enough to be sure it must be the drug.
This is the clearest argument I know for keeping dated records from day one. When a patient can show me that the shedding began in week fourteen and that their fastest weight loss ran from week eight to week twelve, the mechanism stops being a debate and becomes a chart. Without dates it is all impression, and impression reliably blames the most recent thing rather than the thing three months behind it. Zenday or a kitchen calendar — the point is that the dates exist before you need them, because by the time you want them it is too late to create them.
The three things doing the damage
Speed
The single strongest predictor is how fast you are losing. This is well documented after bariatric surgery, where hair shedding is a familiar and expected phenomenon, and it has nothing to do with any drug at all.
Losing two pounds a week is a substantial physiological event. Your body treats it approximately the way it treats a serious illness.
Protein
Hair is largely keratin, and keratin synthesis is not a priority when protein intake is low. On a GLP-1, most people are not restricting protein deliberately — they are simply not hungry, and protein is the macronutrient people drop first when eating becomes effortful.
This is, in my experience, the most fixable contributor and the one most often missed.
Micronutrients
Low ferritin is extremely common in this group, particularly in menstruating women, and it is independently associated with hair shedding. Thyroid dysfunction, low vitamin D, and low zinc are all worth ruling out — not because they are exotic, but because they are cheap to test and treatable when found.
In nearly every case I have worked up, the hair was not a drug reaction. It was an underfed body doing triage.
What to do about it
- Get bloods done. Ferritin, full blood count, TSH, vitamin D, and zinc if available. Do not accept “your iron is normal” without seeing the ferritin number — a normal haemoglobin can sit on top of a ferritin of 12.
- Fix protein first. Most protein guidance for adults losing weight lands somewhere around 1.2 to 1.6 g per kilogram of body weight daily. On a suppressed appetite this needs planning rather than willpower.
- Consider slowing down. If you are losing faster than roughly one to two pounds a week, staying at a lower dose for longer is a legitimate trade. Many people would rather keep their hair and take four extra months.
- Wait. This is the hard one. Telogen effluvium resolves, but the timeline is measured in months, and the regrowth phase — short new hairs standing up at the hairline — looks alarming before it looks good.
The part nobody says out loud
For some people this is the side effect that ends treatment, and I have stopped being surprised by that.
Nausea is invisible. Hair is not. A woman who has lost forty pounds and looks, to herself, worse in the mirror than she did before is dealing with something that a risk-benefit table handles badly. Telling her it is temporary and self-limiting is true and does not touch the actual problem.
What does help, sometimes, is knowing the mechanism — that the body is prioritising, that hair is a metabolically expensive luxury, and that the shed is a lagging indicator of a month that has already passed.